Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

Nitrogen in the Water

If any aspect of nitrogen pollution has a high public and policy profile, it's the effects of excess nutrients on bodies of water, especially in coastal areas. "Because of its high solubility, nitrate quickly escapes to down below the root zone of an agricultural field or forest and into groundwater," says Donald Boesch, a professor of marine science and president of the University of Maryland's Center for Environmental Science. "That makes it difficult and expensive to control."

Nitrogen fertilizer runoff contributes to the formation of algal blooms such as this red tide bloom, which extended more than 100 miles along Florida's Gulf coastline in 2001. Such blooms kill thousands of fish and threaten human health. image credit: NASA

Reactive nitrogen--whether from animal-raising facilities, manufactured fertilizer, septic systems, or other sources--has raised nitrate concentrations in the waterways of most industrialized nations. In Norway, nitrate concentrations in 1,000 lakes doubled in less than a decade. Rivers in the northeastern United States and in much of Europe have increased 10- to 15-fold in the last 100 years.

Where nitrate loading to bays and costal zones increases (rivers tend to be less affected), it can provide such a steady source of nutrients that algae bloom uncontrollably. When the algae die, they sink and decompose, which draws oxygen from the water. If too much oxygen is removed, the water body develops a "dead zone"--an area that can no longer support finfish, shellfish, or most other aquatic life. Perhaps the best-known dead zone is that found in the Gulf of Mexico, which is fed by the nitrate-rich Mississippi River and fluctuates in size from 3,000 to 8,000 square miles. There are also oxygen-starved areas in the Baltic Sea, the Adriatic Sea, the Gulf of Thailand, the Yellow Sea, and the Chesapeake Bay.

Boesch notes that scientists were saying as far back as 1987 that 40% of the nitrogen coming into the system needed to be removed. But so far, he says, programs to reduce reactive nitrogen in the Chesapeake Bay haven't significantly improved the bay's health. And although rivers are generally less susceptible to such algal blooms and oxygen losses, Mallin has found similar effects in North Carolina's blackwater streams, so called because they are rich in organic matter. "Regardless of what we add [nitrate, ammonia, or urea from livestock], it will stimulate algae growth in these blackwater streams," he says.

Reactive nitrogen can also infiltrate drinking water, as nitrates from nitrogen fertilizers and runoff from livestock find their way into streams, rivers, lakes, and groundwater. In the United States, Townsend says, as much as 20% of groundwater sources may exceed the U.S. and World Health Organization limits of 10 parts per million for nitrates. This concentration is also exceeded in many other parts of the world. High concentrations of nitrates can cause methemoglobinemia--or "blue baby disease"--in infants. In blue baby disease, nitrate ions weaken the blood's capacity to carry oxygen. Epidemiological studies have also linked nitrates to reproductive problems and some cancers, including increased risks for bladder and ovarian cancers at concentrations below 10 parts per million.

Nitrogen Sources in 10 Large Watersheds